Add an option (e.g. bool MapperAttribute.AutoIncludeBaseMappings / [IncludeBaseMappingAttribute(bool)] to include base mapping configurations automatically.

[Mapper]
public partial class FruitMapper
{
    [MapProperty(nameof(FruitDto.A), nameof(Fruit.B))]
    private partial Fruit MapToFruit(FruitDto fruit);

    public partial Apple MapToApple(AppleDto apple);
}

When enabled and Apple* extends Fruit*, the configuration of MapToFruit should automatically also be applied to MapToApple.